1. Field of the Invention
The present invention relates to the field of molding articles of rubber or rubber like compositions in which materials of at least two decidedly different hardnesses are integrally combined. More particularly, the invention is directed to the combination of two or more portions or sections, heat curable organic or inorganic rubber materials of decidedly different hardnesses crosslinked to form a single chemically bonded article.
2. Discussion of Related Art
Typically rubber or rubber like parts are formed by placing a homogeneous mixture of the uncured rubber or polymer material in a mold admixed with a catalyst and/or accelerator for curing the article by heat under pressure. This produces a molded article of constant hardness. However, in certain specialized applications, multiple hardness attributes are desirable in a single molded part. An example of such an application where a dual hardness article is required occurs in a vacuum pick-up operation. In that instance, the rubber part is attached to a pick-up fixture where a vacuum is applied and pick-up is actuated. In this type of assembly strength is needed at the fixture/rubber interface (which is not obtainable in a soft material) while softness and conformability is required at the object/rubber interface. In the related art, for many applications where a multiple hardness article is desired, a homogenous material of intermediate hardness is selected thereby compromising both desired attributes of the system.
In the relevant art, composite or laminated article structures have been created utilizing adhesives to fix the parts together or otherwise achieve adhesion between layers or parts of diverse hardness. These adhesives, however, are often limited durability and tend to deteriorate or degrade with time; and eventual separation of the joined sections or laminated layers occurs. In an alternative method of forming multi hardness composite materials, granule or bulk uncured solid materials (such as unvulcanized rubber) are assembled in a mold fashioned in the desired configuration and cured in situ thereby, fusing as in welding, the several diverse materials together to form an integral appliance. The compositions of the uncured compositions may be varied so that the same or concurrent curing conditions promote a different hardness in each composition. While this method can produce an article with multiple hardness sections, the weld does not preserve the integrity of the interface between the diverse compositions and separations readily occur. Such a device is illustrated and described in U.S. Pat. No. 4,448,735 to Huge.
In a different technique, a shaped article of polymeric material is formed using a combination of one polymeric compound which is weldable, e.g., by heat and at least one polymeric compound which can be cross-linked by high energy radiation, for example, electron beam radiation or X-ray. In this technique, the cross linkable polymeric component is distributed within the shaped structure and crossed linked on itself, as desired, based on exposure to high energy radiation. This approach is illustrated by Simm et al in U.S. Pat. No. 4,447,488.
None of these techniques, however, solves the problem of producing a uniform, high-strength homogeneous chemical bond at the interface joining plurality of abutting compositions of diverse hardness such that an integral article structure of multiple hardness attributes is produced. It seems that prior techniques either produce a composite which is of unreliable durability with respect to aging and use or which represents a compromise in the desired hardness properties such that the ultimate goals are not achieved.